In cellular communication, a user equipment existing in a cell accesses a base station to perform communication, receives control information for exchanging data from the base station, and then transceives data with the base station. In particular, since the user equipment transceives data through the base station, in order to transmit data to another cellular user equipment, the user equipment transmits its data to the base station. After having received the data, the base station transmits the received data to another user equipment. Since data can be transmitted through the base station in order for one user equipment to transmit data to another user equipment, the base station performs scheduling of channels and resources for the data transceiving and also transmits channel and resource scheduling information to each user equipment. Thus, in order to perform a device-to-device (hereinafter abbreviated D2D) communication through the base station, each user equipment needs channel and resource allocations to transceive data with the base station. However, according to the structure of the D2D communication, one user equipment directly transceives signals with another user equipment, to which data shall be transmitted, without using a base station or a relay node.
If D2D communication for transceiving data in direct between user equipments is performed in a manner of sharing resources with an existing cellular network mentioned in the above description, each user equipment initiates D2D communication after completion of resource allocation for D2D communication. However, a method of transmitting an initial signal to initiate D2D communication has not been defined yet. Moreover, if the D2D communication for transceiving data in direct between user equipments is performed in a manner of sharing resources with the existing cellular network mentioned in the above description, it may cause a problem, to a user equipment currently performing the D2D communication, that synchronization with a user equipment currently performing D2D communication with the cellular network is mismatched or a problem that synchronization between user equipments currently performing D2D communications with different user equipments is mismatched.